Did anyone ever tell you how badly you suck at prophecies? This unfalsifiable prediction has enormously bad track record: people have called Emacs dead since the 90s, through XEmacs, through every IDE wave since, through Atom, Sublime, nvim, VSCode, Zed, etc.
I'm sure "Anti-AI core community" drinking themselves to oblivion and chugging anti-depressants seeing how many packages for AI integration people keep building¹ and they just can't do anything about it. Bruh, there's literally ongoing threads about built-in LLM support in emacs.devel, the latest one is not even a week old.
Magit being slow is almost entirely git subprocess spawn and output-parsing overhead, which exists in any git porcelain, not something specific to Emacs or elisp². Blaming Emacs for that is blaming the UI for the engine. I use Magit on Mac and it is not slow. It is slower than on Linux, but it ain't unusably slow.
Define "decent harness". First of all you're conflating the layers - ACP is a wire protocol; asking a protocol to be extensible is illiteracy - protocols are meant to be minimal and stable, that's literally their job. eca and agent-shell are ordinary elisp, with hooks, advising, keymaps, same mechanisms every other package uses. If that layer is hookable and advisable, it's extensible in every effing sense; extensibility was never supposed to live in the protocol underneath. Darn, I'm extending eca from inside fucking eca buffer, da fuk you even blabbering about? Whatever, skill issue. Next.
¹ https://www.reddit.com/r/emacs/comments/1uwm3c0/the_state_of...